It is well known that lenses, and in particular organic lenses, require treatment to increase the abrasion resistance of the lens, which is generally the adding of coating.
One known method for coating the surface of the lens, for instance with an anti-abrasive coating, is dip-coating.
In dip-coating, the lens is placed vertically on a basket support, then immersed vertically in a coating solution bath and then withdrawn vertically from the bath.
Vertically dip-coating is very convenient but, when the lens comprises a segment on the surface to be dip-coated, the segment having a curved portion and a straight line portion, there is often a drip line of coating extending from the straight line portion. This drip line comprises an elongated portion the end of which is in the form of a drop-shaped head.
In the case of vertically dip-coating, the drip line comprises a very short elongated portion and a large drop-shaped head which is close to the segment, and thus in the vision area of the lens. Consequently the drip line is obtrusive for the wearer of the lens.
To prevent this drip line, one known method of dip-coating is to withdrawn the lens with a tipping movement of the lens. Thereby, the lens is immersed vertically and during the withdrawing, the lens is tipped from a vertical position of the lens until, when fully withdrawn from the coating solution bath, it reaches a horizontal position of the lens.
Such tipping of the lens prevents the formation of a drip line but provides a coating thickness on the dip-coated surfaces of the lens which is too thick, and furthermore there is often a thickness variation of the coating between the top and the bottom of the lens. This thickness variation can lead to cracks on the surfaces of the lens when the coating on the coated-lens is cured.
The invention is directed to a method of dip-coating which limits both drip lines and thickness variations on the surface of the lens.